I've recently started organising my work into projects (yes, I should have done this years ago!) but I do find that I need to switch between projects very frequently. It would be really great if the "open project" action gave a choice of opening a new window for the new project without closing the window for the existing project: that is, having multiple windows open, one per project.

There are plenty of other tools that work this way so it would be a very natural extension.

Thank you for the feedback.
I have logged your request on our issue tracking tool and it will be analyzed and implemented in a future version.

Meanwhile, as a workaround, it is possible to start multiple instances of Oxygen (each can have its own project) via the command line startup script with a modification of the script.
There is a catch, the same set of options is used by all the instances and they all write these options to the same file at quitting time. This means the options file keeps getting overwritten, so the last instance you quit is the one that will have persistent options.

For Mac/Linux, edit the oxygen.sh script and after line 85 (set -- \) insert a new line with:

You may also want to adjust the heap memory (-Xmx1g argument) depending on how large are the projects. 1GB (1g) should be enough for the average project, but if you have something large, you can increase it to -Xmx2g (2GB) or -Xmx4g (4GB). Make sure you have enough system RAM to accommodate the memory used by multiple running instances.

After this change, execute the oxygen startup script multiple times (one per instance). There is also the possibility to specify the Oxygen project (.xpr) file as argument of the script (so it starts directly with that project), or you can switch projects afterwards.